Tissues are formed and maintained by stem cells that produce both daughters that undergo self-renewing proliferation and daughters that differentiate. The mechanisms by which the choice between the self-renewal/ proliferative fate and the differentiated fate are made are not well understood in any system. However, disruption of the decision can cause stem cell loss, resulting tissue depletion, and lead to cancer. Our long-term goal is to understand how the proliferation vs. differentiation decision is made in the C. elegans germline. The C. elegans germline is the major model system for tissues where there are a larger number of stem cells that divide and differentiate through symmetric divisions, in contrast to the more widely studied systems with a small number of stem cells and differentiation through asymmetric division. The GLP-1 Notch signaling pathway induces the germ cell proliferative fate and represses three redundant pathways that promote the meiotic cell fate: the GLD-1 pathway, which acts in translational repression; the GLD-2 pathway, which acts in translational activation; and a third pathway whose existence has been revealed through genetic analysis but no gene products have been identified to date. Notch signaling in mammals is also important in stem cell self-renewal and oncogenic Notch activation can lead to cancer. Project goals address major unanswered questions in the field and include: (1) Determining whether the proliferative zone population is composed of only stem cells or both stem cells and proximal transit amplifying cells. (2) Identifying transcriptional targets of GLP-1 signaling for the proliferative fate and determining regulatory relationships with the three meiotic entry pathways. (3) Identifying the GLD-1 targets that are translationally repressed to promote meiotic entry. PUBLIC HEALTH RELEVANCE: The proposed research investigates the stem cell versus differentiation decision, which is an essential part of animal development and adult tissue homeostasis. Disruption of the decision can cause stem cell loss, resulting in tissue disfunction, and can lead to cancer.